Recently, a plasma display panel (hereinafter referred to with its abbreviation, “PDP”) and a ferroelectric liquid crystal display have drawn attention as a display device with which display in a large screen, great display capacity, and multiple-gray-level display can be realized.
For example, a PDP generally adopts the time division gray-scale display method as follows: in advance, one field (or called as one frame in some cases: either of the same defines 60 Hz cycle) period is divided into a plurality of 8 to 12 sub-field (sub-frame) periods that differ in the length of the emission period, and independent ON/OFF states of the sub-field periods are selectively combined, so that the gray-scale display should be carried out due to the effect of accumulation sensed by eyes (afterimage effect).
A concrete example of realizing 256 gray levels by the foregoing time division gray-scale display method is shown in FIG. 61. In this example, one field period is divided into 8 sub-field periods SF1 through SF8, then each of the sub-field periods SF1 through SF8 is divided into an addressing period and a display period, and the relative ratio of the lengths of the display periods is set to 1:2:4:8:16:32:64:128 (2n; n=0 to 7). By selectively combining independent ON/OFF states of the respective sub-field periods thus set, 256 gray levels can be realized.
However, in the case of the foregoing time division gray-scale display method, to display a gray level “127” for example, the state is ON in the sub-field periods SF1 through SF7 as shown in FIG. 62, thereby resulting in that the emission period (indicated by hatching in the figure) of PDP comes concentratedly in the first half of the one field period. Besides, to display the gray level “128”, the state is ON only in the sub-field period SF8, thereby resulting in that emission period comes concentratedly in the latter half of the one field period.
Therefore, as shown in FIG. 63 as an example, in the case where an object 112 with a brightness of the gray level “128” moves upward in a background 111 with a brightness of the gray level “127”, the observer, since following the object 112 with eyes, captures motion from an image 112a to an image 112b as the object 112.
Here, a pixel located at a lower part of the contour of the image 112a displays the gray level “128” according to the ON state in the sub-field period SF8, and subsequently displays the gray level “127” of the background 111 after the image 112a moves upwards according to the ON state in the sub-field periods SF1 through SF7. Thus, the pixel located in the lower part of the contour of the image 112a is successively ON in the sub-field period SF8 and the sub-field periods SF1 through SF7, and consequently the pixel appears to the observer as if it would display a gray level “255”.
On the other hand, a pixel located at an upper part of the contour of the image 112a moving has displayed the gray level “127” of the background 111 until before the image 112a moves thereto, and therefore, the pixel is turned OFF in the sub-field period SF8 after it has been ON in the sub-field periods SF1 through SF7. Then, with the image 112b moving thereto, the pixel necessarily remains OFF in the sub-field periods SF1 through SF7 until it is turned ON in the sub-field period SF8, to display the gray level “128”. Thus, the pixel located at the upper part of the contour of the image 112b moving thereto is successively OFF in the sub-field period SF8 and the sub-field periods SF1 through SF7, and consequently the pixel appears to the observer as if it would display a gray level “0”.
Thus, there occurs a phenomenon in which the gray level (0, 255) different from the gray level that should be seen (128) actually appears to be seen in a contour of the object 112 (this phenomenon is hereinafter referred to as “motion picture pseudo contour”).
In most cases, the motion picture pseudo contour is a phenomenon in which upon motion of a picture with smooth gray level gradation, a belt-form virtual image having a luminance or a chromaticity that does not originally exist in a picture is recognized, giving the observer a strong impression of deterioration of image quality, or a phenomenon in which, in an object with smooth gray level gradation, gray level interference with a remarkable peak occurs in a continuous space with respect to a specific gray level shift. Since the interference is spatially continuous, this results in that a contour that should not exist is seen by the humans' eyes.
In other words, this motion picture pseudo contour is a new image-quality-concerned problem that has not occurred to CRT displays but occurs to PDPs and the like that adopt the time division gray-scale display method, and it can be defined as: “distortion of an image observed when the point of view travels over a display device screen; often occurring to a contour part of a motion picture displayed in gray scale; generation thereof depends on a product of a length of an emission period of a pixel and a view point travelling speed, as well as on non-uniformity of emission in terms of time, and causes disorder of gray levels and colors”.
Incidentally, such a principle of generation of the motion picture pseudo contour is explained by Mikoshiba et al., “Dynamic False Contours on PDPs-Fatal or Curable?”, IDW′96 (Kobe International Meeting in 1996, 11, 27–29). It is also explained in “Consideration on Improving Motion Picture Quality of PDP with use of a Sub-Field Method” (Keiji ISHII et al., Technical Report of the Institute of Electronics, Information and Communication Engineers, Vol. 97, No. 336, issued on Oct. 23, 1997).
On the other hand, to alleviate the motion picture pseudo contour, for example, the foregoing “Consideration on Improving Motion Picture Quality of PDP with use of a Sub-Field Method” directed attention to that the amplitude of the motion picture pseudo contour that is a difference between the gray level of the motion picture pseudo contour and the original gray level increases in proportion to the length of a display period in a sub-field period, and proposed that the amplitude of the motion picture pseudo contour is decreased by dividing the sub-field period including a long display period thereby increasing the number of sub-field periods having short display periods.
FIG. 66 illustrates an example of division into sub-field periods. In this example, the number of sub-fields is increased from 8 to 10 based on the following two types of division formulae:128+64=64+64+32+32128+64=48+48+48+48
The foregoing document describes that image quality can be improved by, apart from the method which increases the number of the sub-field periods, the method which compresses the display period of each sub-field period, the method which optimizes the arrangement of the sub-field periods, or the method which adaptively controls emission patterns by signal processing to select an optimal emission pattern of each sub-field period.
Furthermore, for example, the Japanese Publication for Laid-Open Patent Application No. 39828/1998 (Tokukaihei 10-39828 [Date of Publication; Feb. 13, 1998]) discloses a method which inserts a correction gray level value or a correction pulse in a part of gray level shift at which the motion picture pseudo contour is generated.
The technique of the publication is intended to alleviate the motion picture pseudo contour of the video in a half-tone display method and device for performing half-tone display by the frame time division method, and as explained with reference to FIG. 61, the technique relates to the half-tone display method which, to display an image, provides a plurality of emission blocks in each frame and displays half-tones with combinations of the emission blocks.
In the foregoing half-tone display method, in the case where light emission patterns of light emission blocks of respective pixels change in successive frames, image information is compared between two frames, and the light emission block preliminarily determined for luminance adjustment is added or subtracted to or from each pixel where light emission state changes, according to the state of the change.
More specifically, as shown in FIGS. 64(a), 64(b), and 64(c), when a display picture is scrolled from the left side to the right side at a rate of 1 coordinate/F (frame) along an x-axis on retinas in a state in which halftone levels with respective luminances K(x) of 128 and 127 are adjacently displayed, a dark line (DL) is generated in the boundary part between the halftone levels 128 and 127, that is, at coordinates of x=4 on retinas that follow the display picture moving. The dark line (DL) is expressed as L(1)=L(3)>>L(2), using a stimulus quantity L(x) on retinas.
In this case, as shown in FIGS. 65(a), 65(b), and 65(c), a stimulus value ΔL(4) according to an equivalent pulse EPA (light emission block: sub-frame) that is derived so as to satisfy the following formula is added to pixels where the dark line (DL) is generated:L(1)≧L(2)+ΔL(4)≧L(3)Then, as shown in FIG. 65(c), it follows that the stimulus value L(x) on retinas is added by the stimulus value ΔL(4) in L(2) of the boundary part between the halftone levels 128 and 127 and the moving picture pseudo contour (color pseudo contour) of the video is suppressed.
On the other hand, in the case of a ferroelectric liquid crystal display device, it is possible to adopt a time division gray-scale display method identical to the aforementioned one, taking advantage of the characteristic of ferroelectric liquid crystal in that the orientation of molecules abruptly switches between two directions when an applied electric field crosses a threshold value.
For example, the Japanese Publication for Laid-Open Patent Application No. 152017/1995 (Tokukaihei 7-152017 [Date of Publication: Jun. 16, 1995]) discloses a method in which ultrafine particles are dispersed in a layer of ferroelectric liquid crystal so that a micro domain whose transmittance is caused to vary with a voltage applied is generated around each ultrafine particle, and gradation control is executed by any one of the pulse voltage/amplitude modulation method, the pixel electrode division method, and the time division method, or combination of some of the same.
However, although the turbulence of luminance in the motion picture pseudo contour increases in interrelation with a motion speed of a picture, as will be described later in the “Description of the Preferred Embodiments” section, the foregoing conventional method for alleviating or correcting the motion picture pseudo contour does not take the motion speed of a picture into consideration at all.
More specifically, the foregoing “Consideration on Improving Motion Picture Quality of PDP with use of a Sub-Field Method” describes the method which increases the number of time divisions, the method which compresses the display period of each sub-field period, the method which optimizes the arrangement of the sub-field periods in terms of time, and the method which adaptively controls emission patterns by signal processing to select an optimal emission pattern of each sub-field period, but these methods are methods that merely improve the motion picture pseudo contour, uniformly, irrespective of the motion speed of a picture, an individual magnitude of gray level turbulence, etc.
Furthermore, the method for correcting a motion picture pseudo contour disclosed by Tokukaihei 10-39828 discloses a method that merely adds/subtracts a correction-use light emission block that is unchanged even with variation of the motion speed, with respect to a pixel at which the motion picture pseudo contour is generated. The foregoing document does not propose a correction method responsive to the motion speed of a picture.
Furthermore, Tokukaihei 7-152017 does not mention the problem of the motion picture pseudo contour at all.
Therefore, the foregoing conventional schemes regarding the motion picture pseudo contours have not gone farther from the level of mere decrease of the motion picture pseudo contour, as shown in the graph of Tokukaihei 10-39828 that shows a result of correction. Thus, there still remain problems relating to precision of correction.
On the other hand, the method that increases the number of the sub-field periods resulting from division of one field as proposed by the “Consideration on Improving Motion Picture Quality of PDP with use of a Sub-Field Method” achieves an effect in decreasing the motion picture pseudo contour and realizing high quality display, but undergoes the following secondary problem, since a time for one scanning cycle has to be shortened.
Taking into consideration the response time of the emission elements per se, the time for turning on/off gates provided in each element, and blank periods in display, a waiting time is required to some extent other than the time for scanning. In the case of an overall-flush-type PDP, gates are scanned in a dark state, and overall light emission is executed after a certain necessary time has passed after the scanning was thoroughly completed. For this reason, as the number of sub-fields increases, the limitations on the time required for the light emission process may possibly become not tolerated, as the number of scanning lines increases. In short, control of operation timings of the device becomes difficult.
Furthermore, with regard to circuitry, signals are made to have high frequencies as the number of sub-fields increases, and hence, the power consumption of the device tends to increase. This is because that the power consumption is generally proportional to a frequency used. More specifically, the number of times of discharge inside a circuit is generally proportional to the frequency, and proportionally the average current increases as well. Therefore, if resistance components in the circuit cannot be sufficiently lowered, consumed power increases as the current increases.